Rihanna had another unwanted visitor swing by her pad, and this one might have her thinking about digging a moat.

Law enforcement sources tell TMZ … cops were called to one of Rihanna’s L.A.-area homes last week after getting a call reporting a trespasser on the grounds pretty early in the morning — around 7 AM or so.

Our sources say cops arrived with lights and sirens, but when they showed up to the crib, they were notified the alleged perp was long gone. We’re told Rihanna’s security informed cops the guy was seen hopping over one of her perimeter walls, but clearly didn’t stay too long. The police took a report and are investigating.

RiRi is no stranger to unwanted visitors showing up to her homes — she’s had quite a few run-ins over the years … enough that her security responds in a flash, and alerts police almost instantly.

BTW, Rihanna wasn’t home during the incident — while she owns it, we’re told she actually rents out the property.